1. Field of the Invention
This invention relates to a polyester laminated film with sealing ability, and in particular, to a polyester laminated film that has good heat-sealing properties, the sealed portion of which can be peeled off readily by the use of an appropriate amount of force. The film can be used as a film for wrapping and as a film for many industrial uses.
2. Description of the Prior Art
Polyester films (here and below, the word "film" also includes the meaning of "sheet") that have excellent properties such as mechanical strength, heat-resistance, cold-resistance, resistance to chemicals, insulating qualities, stable dimensions, a uniform surface, and transparency are used in a variety of applications such as film for wrapping, tape for electrical insulation, magnetic tape, photographic film, and tracing film. Polyester films are particularly suitable for the wrapping of products, such as for the wrapping of food, because polyester film has no odor, and does not absorb the odors of the food. For such films, films with heat-sealing ability are preferable. However, in general, the melting point of polyester films is high, so it is difficult to perform heat-sealing of the film if polyester film made with a typical polyester, such as polyester terephthalate, is used. Therefor, these polyester resin films are used as the base film, and a sealing layer with a lower melting point is formed on its surface. As resins that can be used for such heat-sealing layers, there are polyolefin resins, vinyl resins, polyester resins, etc. The adhesiveness of polyester resins to a base film made of polyester is particularly satisfactory, so polyester resins are preferable for use in heat-sealing layers.
It is desirable that the sealed portion of such laminated films have the quality of toughness after it has been sealed by the use of heat. Here, the word "toughness" will be used to mean sufficient strength, with permanence, and with the qualities of flexibility and lack of fragility. If the sealed portion does not have toughness, then, for example, if the opening of a bag is heat-sealed to close it, then when the opening of the bag is held in the fingers and pulled to open it, the sealed portion will not peel off properly, but will tear midway, and the bag will tend to be ripped without satisfactory peeling open of the sealed portion, which is a disadvantage.
This kind of laminated film with a heat-sealing layer generally has unsatisfactory slipperiness, and even if a lubricant is added to the base film, it is difficult to wind the film into a roll, and workability is poor. Japanese Laid-Open Patent Publication No. 56-166065 discloses a method by which the slipperiness of a film is improved. In the method, there is addition of an additive consisting of particles the diameter of which is larger than the thickness of the adhesive layer. By the addition of this additive, minute protrusions are formed on the surface of the heat-sealing layer, and this increases the slipperiness of the film, improving workability. However, when a thin film of a metal or metal oxide, such as on aluminum, is deposited on the surface of such laminated films, it is not possible to form a uniform thin film because of the projections on said film surface. Also, the thin film that is formed on top of the particles will be removed when the film obtained is being rolled up or the film comes into contact with a roller surface at the time of manufacture or during secondary processing, resulting in pinholes at the portion that was removed. The gas-barrier properties of the film are worsened by the presence of a number of pinholes, and such a film cannot be used in products that must act as a gas barrier, such as a lid for the sealing of containers to be used for food storage.